Movable micromachined silicon structures have numerous uses. They frequently are used as sensors and actuators, and for signal processing. For example, the fingers of a movable mass may be positioned between stationary plates to form a differential capacitor. When a force is applied to the structure, the movable mass moves relative to the plates, changing the capacitances and inducing an amplitude modulated signal onto the mass. The induced signal, which is proportional to the magnitude and duration of the acceleration, may be output for further processing. An example of such a device is described in U.S. Pat. No. 5,345,824, entitled "Monolithic Accelerometer," which is incorporated herein by reference.
In designing a micromachined electro-mechanical structure, it is important that parasitic capacitances be minimized. One technique that has been developed to combat this problem is to place a conductive ground plane between the substrate and the movable mass, so as to isolate the mass from the substrate. U.S. patent application Ser. No. 08/347,702, entitled "Conductive Plane Beneath Suspended Microstructure," filed Dec. 1, 1994, by R. Howe, et al., which is commonly assigned and is incorporated herein by reference, describes an improved ground plane, which is exposed rather than covered by a dielectric, and a method for manufacturing such an exposed ground plane. Through that structure and method, parasitic capacitances are greatly reduced. However, precise positioning and substantially vertical etching of the edges of the dielectric surrounding the exposed portion of the ground plane remains difficult to achieve.
The present invention overcomes these problems as will be shown in the remainder of the specification referring to the attached drawings.